One conventional method for measuring vacuum or negative pressures in a range of from 0.1 millibar to 1 bar (ambient atmospheric pressure) is by thermal conductivity. The temperature of an electrically heated wire depends upon heat loss to the surrounding gas. As gas pressure is reduced and its thermal conductivity decreases, the temperature of the heated wire increases. Usually, the heated wire is connected as one part of a Wheatstone bridge, so that electrical change in resistance associated with heating of the wire is related to the vacuum to be measured. Alternatively, the heated wire is replaced by a thermistor which is similarly electrically heated, but which displays much larger resistance changes in vacuum.
The thermal conductivity gauge is commonly known as a Pirani gauge. The thermal conductivity method presents two problems: First, the heated sensor consumes power, requiring heavy batteries or a connection to the AC mains power supply. Second, the sensor requires a warmup time for reaching thermal equilibrium before measurements may commence. In addition, this method usually requires operator intervention and adjustment in order to balance the bridge initially upon powering up.
It is known that the presence of air or other gases surrounding a mechanical vibrator exerts a damping effect upon the vibration. As the pressure of the surrounding gas is diminished, the damping effect is reduced. A vacuum gauge known to those familiar with vacuum techniques as the Langmuir gauge (see, Beckman, J.O.S.A., 16, 276 (1928)) utilizes this effect. A fine quartz fiber is anchored at one end and the other end is free to vibrate. Vibration is excited by striking the fiber (internally) and the time for the free vibrations to decay to half amplitude is monitored. Essentially, the decay time is as an indicator of vacuum. As the gas pressure is reduced, the decay time of the vibrator increases.
Miniature quartz tuning forks intended for timing control for wristwatches have been suggested as vibrators for this purpose. Measurements of the effect of changes in gas pressure on the Q (or alternatively on the reciprocal of Q) of these commercial devices are disclosed in a paper by Christen entitled "Air and Gas Damping of Quartz Tuning Forks", Sensors and Actuators 4, pp. 555-564, (1983). Christen's disclosure is a report of laboratory measurements, and he offers no means for measuring the Q which would suggest a device or sensor which could be reduced to a practical, portable instrument for vacuum measurement.
Measurement of Q in an electrically oscillating circuit is well known. Generally, there are two alternative methods for measuring Q. The first method is to measure the time for the oscillations to decay (e.g. to one half amplitude) after the driving oscillator has stopped. This is the method of the Langmuir gauge. A second method is a measurement of the bandwidth of the vibrator by driving it with a variable frequency oscillator and tuning over the frequency band of the vibrator. Neither of these two methods lends itself to a simple direct measurement suitable for a commercial gas pressure gauge.